A treasure trove of rare gold coins that were discovered by a California couple out walking their dog last year went on sale Tuesday, with the first coin fetching $15,000 that will be contributed to turning San Francisco’s Old Mint into a museum.

The origin of a treasure trove of gold coins found on private property in Northern California remains a mystery after U.S. Mint officials said Tuesday that the coins have no apparent link to thefts at their facilities.